Fasteners of multiple designs, including nuts, screws, clips, rivets and many other varieties, have been made available depending on the particularly encompassed application. A common feature thereof lies in the particularly stringent requirements associated herewith.
Indeed, during their life, any fasteners, including notably unthreaded and threaded fasteners, are notably submitted to harsh mechanical conditions: they are often stretched, twisted and bent. Fasteners are further generally submitted to various aggressive environmental, such as vibrations, thermal cycles and/or chemical attacks, which can alter their mechanical performances over time, and, in the worst scenarios, cause fasteners to be literally “lost”.
A class of fasteners of particular importance is threaded fasteners that face with additional specific problems. When threaded fasteners are tightened, we pump energy into them, and, after we let go, this energy is held therein by friction constraints. Typically, these ones are concentrated to a large extent in the fastener threads, which yet often represent the most delicate portions of the fastener as the result of their fineness. Aggressive environmental factors as above recited can cause threaded fasteners to loose all their preload and literally be lost (“loosening” problem). Somewhat related to loosening is thread stripping, another failure which occurs when threaded fasteners are over-tightened; thread stripping is characterized by a deformation (alteration) of the fastener threads, typically resulting in a decrease of performance.
Certain fasteners, including certain unthreaded and threaded fasteners, can have complex designs, and shaping/machining them from an appropriate material can be a tough matter. In case of threaded fasteners, forming fine and regular threads has proved to be particularly uneasy.
Material selection is crucial for fasteners.
Metal fasteners present however a certain number of disadvantages. Metal fasteners are heavy which is of a disadvantage in applications where weight is a concern, such as notably in aircrafts applications. Metal fasteners are in general also prone to corrosion and are electrically conductive. Machining complex shapes from a metal, as well as forming fine and regular threads, is also a tough matter.
Thus for several reasons, as listed above, polymeric materials are useful metal replacements.
However, it is important that said fasteners are made from polymeric materials that are easy to process into fasteners and that said polymeric materials are able to provide the required level of mechanical properties, in particular having especially an excellent balance of stiffness and ductility, a high elongational strength, a high flexural strength and, last but not least, a high torsional strength (or torque) which is related to the inherent shear strength of the material. Indeed, for many fasteners, including but not limited to threaded fasteners, load bearing ability is typically a function of the shear or torsional strength of the material of construction.
It is known that carbon fiber-reinforced polyetheretherketone (PEEK) compositions have already been used to replace aluminum, brass, steel and stainless steel fasteners in a wide variety of applications such as notably in aerospace applications.
It is known that in the manufacturing of fasteners, using different processing technologies, such as notably injection molding or extrusion, the crystallization rate of said PAEK polymer plays an important role. A slightly slower crystallization rate can already result in the manufacturing of fasteners, in particular in small portions of said fastener, having for instance non-uniformity defects such as notably appearance and color non-uniformities, as well as skin-core variation effects wherein the threaded screw, for example would be more amorphous near the wall and more crystalline neat the center. This leads to the peripheral section of the screw having the limiting shear strength in the molding overall.
There remains a continuous need for fasteners made of polymeric compositions wherein said fasteners features an excellent balance of stiffness and ductility, high torque, high practical toughness (high tensile elongation), high elongational capability in tension and under impact loads, high strength, high chemical resistance, light weight, and uniform crystallinity which can be easily formed by melt-processing techniques, such as extrusion or injection-molding, including when the parts of concern have a complex shape and/or very thin portions (e.g. threads, or when an extruder with small orifices must be used). Last but not least, the polymeric materials need to be as low in specific gravity as possible, due to the criticality of light weight and energy efficiency of the current and future generations of aircraft. In these cases, the use of a glass fiber reinforced resin with a relatively high loading of glass reinforcement (i.e. 20% or more) can become disadvantageous from a unit weight and mobility standpoint as these reinforcements significantly increase the density of the composition relative to the corresponding unfilled polymer. Carbon fiber can mitigate this effect due to its lower density relative to glass fiber, but on the other hand carbon fiber-reinforced plastics have an electrical conductivity issue which can lead to galvanic corrosion of mating aluminum structures exist. This limits the range of utility of carbon fiber-reinforced PEEK. Reinforced plastics like PEEK reinforced with glass fiber and carbon fiber, for example, suffer from the fact that the composition has non-uniform properties over the various locations of the part, depending on how the fibers are oriented and how the finished part is fabricated. Strength and stiffness properties are very high in the direction of flow or direction of alignment of the fibers and much weaker properties are realized perpendicular to the orientation of these fibers. The strong anisotropy just mentioned also leads to warpage issues in injection molded parts as different portions or dimensions of the part may shrink differently depending on the state of fiber alignment in that particular direction. There is therefore a need in the art for reinforcements or fillers that do not impart this kind of high anisotropic character to the compositions used in fasteners used for aircraft and aerospace applications.